Recreation vehicle makers are riding a boom fueled by younger buyers, but their share prices have rallied too far

For anyone who has gotten stuck on a mountain road behind a massive recreational vehicle, get used to it, there are a lot more on the highway.

Recreational vehicles, ranging from bus-sized motor homes to retro trailers, have been a boom-and-bust industry since they first became popular in the early 1970s. Now a wave of retiring baby boomers and a surprisingly young new fan base have sent U.S. unit sales above their housing boom peak. Shares of the two leading manufacturers of RVs, Thor Industries and smaller Winnebago Industries,...